A shift register unit in related arts includes a signal output terminal for outputting a driving signal. The shift register unit controls a row of thin film transistors (TFTs) to be turned on when the driving signal is at a high level and to be turned off when the driving signal is at a low level.
A LCD generally employs a way of progressive scanning, in which when a certain row or column is scanned, a corresponding shift register unit outputs a driving signal at a high level, and each of the other shift register units outputs a driving signal at a low level. It can be seen that, for one shift register unit, the driving signal thereof is at a low level for most of time.
During a period when the driving signal is at a low level, the driving signal is highly inclined to be influenced by an inputted clock signal, so that a noise may be generated. To suppress the noise, the shift register unit generally includes a lowering signal TFT for pulling down the driving signal during a period when the driving signal is at the low level. A junction node connected to the gate of the lowering signal TFT controls the lowering signal TFT to be turned on, thereby the level of the gate driving signal at the signal output terminal can be pulled down.
A problem existing in the shift register unit of the related art is that, the node connected to the gate of the lowering signal TFT is generally kept at a high level for most of time. and in this way, the lowering signal TFT remains conductive for most of time, which makes the threshold voltage of the lowering signal TFT generate a relatively large offset. If the threshold voltage of the lowering signal TFT is boosted constantly. it may cause the lowering signal TFT to fail to turn on and thereby to fail to serve to suppress the noise, thus performance of the shift register as a whole will be influenced.